In the formation of many plastic articles, plastic sheet material is usually passed between die rolls, one or both of which may have a pattern negative for producing a pattern on the final sheet plastic product. Typically however, the formation of plastic products according to these conventional processing techniques depends upon the use of previously formed sheet stock plastic material. While the use of previously formed sheet stock material is typically not objectionable when most plastic products are produced, there are some significant disadvantages when these conventional product-forming techniques are employed to produce plastic products having contoured e.g., beveled) peripheral edges.
For example, in the production of floor mats of the type which are customarily placed between a chair's legs and the floor's surface so as to protect the latter from the marring effects of the former, it is conventional to first form a sheet of plastic material and then "punch out" (i.e., stamp) individual mats from the previously formed plastic sheet. Obviously, this technique toes not lend itself to the formation of contoured or beveled peripheral edges which bound the mat. Instead, when beveled edges are desired, it is customary to employ a router or like edge-forming tools so as to, in essence, grind, cut or otherwise shape the mat's edge to the desired contoured configuration. This conventional technique, while effective to form contoured edges, sacrifices the visual appearance of the edge (not to mention its labor intensity which contributes to increased costs and/or slower production rates for such chair ma&:s). Thus, the peripheral edges of mats formed in this conventional manner usually are marred due to the abrasive effects of the mechanical router.
The problems mentioned above could be solved by batch molding individual mats using suitably configured molds--i.e., molds which have the desired contoured edges of the resulting mat product. However, it is well recognized that batch molding of chair mats is a cost-prohibitive solution and thus, is not viable to mass produce chair mats at reasonable production costs.
It would therefore be desirable if contoured edges could be "molded" in a continuous in-line manner so as to enhance the visual appearance of the edges. It would also be very desirable from a production economic point of view if such in-line molding could be accomplished simultaneously with the extrusion of a thermoplastic material so as to produce, in a unified step, both the mat (or at least a preform of the mat) and the mat's contoured or beveled edges. The methods according to this invention achieve these desired results.
According to this invention, a plastic product (for example, a chair mat) having continuous contoured (e.g beveled) peripheral edges may be formed by using a novel pattern roll. An interior region of a cylindrical surface portion on the pattern roll is bounded by means of a continuous "rib" which corresponds to the outline of the mat and also has surfaces corresponding to the desired contoured or beveled edge of the mat.
A nip is formed between the pattern roll and a platen roll. Upon relative rotation of the pattern roll in opposition to the platen roll, discrete volumetric portions of the bounded interior region will sequentially continuously be presented to the defined nip. Simultaneously with the rotation of the pattern roll, a molten thermoplastic material is extruded into the volumetric portions of the bounded interior region which are sequentially continuously being presented at the nip. This extrusion/rotation thus causes the molten thermoplastic material to fill the presented volumetric portions of the bounded region. As may be appreciated, when this technique is practiced on a continuous, in-line manner, the bounded region will eventually form an outline of the plastic product and will, moreover cause the thermoplastic to be flowably compressed in the defined nip. This compression, in turn, allows the contoured edges to, in effect, be molded for each sequential volumetric presentation of the bounded region. In other words, the method of the present invention employs thermoplastic extrusion techniques in combination with an in-line molding technique so as to form a plastic mat, for example having the contoured edges.
A plastic product (e.g., a chair mat) is thereby produced with contoured or beveled edges without the need to first form a sheet of plastic stock and then subject the stock to after-treatments such as, cutting, edge forming and the like. That is, the method of the present invention produces contoured or beveled edges on a plastic product simultaneously with the formation cf the product itself (or at least simultaneously with the formation of a product preform).
The novel pattern roll which may be employed in the product-forming method described briefly above is fabricated from a cylindrical base roll and a cylindrical sleeve. Specifically, a recessed groove having at least some circumferentially and longitudinally (i.e., relative to the base roll's axis) extending groove sections is formed in the base roll. Corresponding circumferential and longitudinal rib segments are formed from the sleeve by removing unwanted regions thereof. Beveled surfaces providing a negative image of the beveled edge on the article to be produced are then formed on these circumferential and longitudinal rib segments.
Important to the present invention is that the beveled surfaces on the circumferential rib segments are formed at an angle A.sub.1 less than an angle A.sub.2 of the beveled surfaces formed on the longitudinal rib segments. Notwithstanding this angular difference, a plastic product having a peripheral beveled edge of substantially constant geometry may be formed.
Adjacent ones of the circumferential and longitudinal rib segments may be joined by means of a radius rib segment which also includes a beveled edge continuous with the beveled edges of the circumferential and longitudinal rib segments which it joins. However, due to the angular difference of the beveled edges formed on the circumferential and longitudinal rib segments, the beveled surface formed on the radius rib segment presents a smooth angular transition surface between the angles A.sub.1 and A.sub.2.
Other advantages and aspects of this invention will become more clear after careful consideration is given to the detailed description of the preferred exemplary embodiments of the invention which follow.